Electric double layer capacitors have advantages that the electric double layer capacitors can be used at a wide range of temperatures and have high power density. In order to enhance the energy density of the electric double layer capacitors, it is necessary to increase the working voltage of the electric double layer capacitors. Widely used conventional electrolytic solution is an organic solution (non-aqueous electrolytic solution) which contains chain carbonate or cyclic carbonate as a solvent and a quaternary ammonium salt as a supporting electrolyte. When an electric double layer capacitor including an electrolytic solution containing chain carbonate and/or cyclic carbonate as a principal solvent is used under high voltage conditions, the electric double layer capacitor has various problems such that the electrolytic solution is readily degraded to generate gases.
When a capacitor including electrodes made of an alkali-activated carbon has an electrolytic solution containing chain carbonate and/or cyclic carbonate as a principle solvent, the capacitor has a problem that the solvent is degraded and a large amount of gas is therefore generated if the capacitor is used under high-voltage and high-temperature conditions. That is, a capacitor including such an electrolytic solution is difficult to be used under a high voltage conditions or a high temperature, because the solvent is promoted in degradation thereof to generate decomposed gases including carbon dioxide gas.
In view of an increase in electrical conductivity and/or capacitance change, it has been attempted that various types of solvents or solvent mixtures are used for electrolytic solutions for the electric double layer capacitors. In order to put the electrolytic solutions to practical use, the electrolytic solutions need to meet various physical requirements such as low-temperature properties, ability to dissolve a salt (the dielectric constant of a solvent), safety, degradability, boiling point, and cost. Chain carbonate and cyclic carbonate are examples of a solvent, widely used for the electrolytic solutions, meeting such requirements.
A known example of an electrode material is an activated carbon, prepared by treating a graphitizable carbon material by alkali activation, having high capacity (Japanese Unexamined Patent Application Publication No. 09-275042). However, a capacitor having an electrode made of the above activated carbon has a lot of defects when composing cells thereof, because the above activated carbon generates more gases due to degradation of a solvent than a steam-activated carbon. In particular, when a capacitor including electrodes made of an alkali-activated carbon has an electrolytic solution containing a carbonate solvent, the capacitor has a problem that the solvent is degraded and CO2 of gas is therefore generated if the capacitor is used under high-voltage and high-temperature conditions.
Examples of a technique for preventing decomposition gases, such as carbon dioxide, include a technique using an electrolytic solution, containing a solvent containing γ-butyrolactone or γ-valerolactone, for capacitors (Japanese Unexamined Patent Application Publication No. 2001-217150); a technique using an electrolytic solution containing fluorobenzene (Japanese Unexamined Patent Application Publication No. 2004-6803); and a technique using an electrolytic solution containing diphenyl (Japanese Unexamined Patent Application Publication No. 2004-146610).
In an electric double layer capacitor including electrodes made of an alkali-activated carbon prepared from a graphitic material, the above techniques are ineffective in preventing gas from being generated due to the degradation of an electrolytic solution.